It was about new states having the right to slavery. Both sides of the war had slavery, and the union only abolished it in confederate states. The eventual amendment kept slavery for prisoners so you know it wasn't anti-slavery. They felt new states without slaves wouldn't vote in their best interest. To reduce it to slavery shows an inability to relate the events to present times.

For instance PR isn't a state because both sides think they will support the other. Or Gerrymandering is done union wide to ensure states vote for the party that implements it.